1. Field of the Invention
The present invention applies in particular to measuring the azimuth of the deep part of a drill-hole when the drilling has been deliberately or involuntarily done on a curve. The objective of this measurement is, in particular, to discover the position of the drilling tool and the direction in which it is advancing.
2. Description of the Prior Art
The trajectory of a drill-hole is geometrically determined by two parameters which are in principle defined at each point on the curved axis of the hole, which change along the length of the hole and integration of which over this length of the entry of the hole up to a point on the axis of the hole gives the coordinates of this point. The first of these parameters is the inclination of the axis at the point in question relative to the vertical. The second is defined only if the inclination is not null. It is the azimuth of this axis, that is to say the orientation of the vertical plane through the axis of this section relative to a horizontal reference direction which is in general either magnetic north or geographic north. Measuring it would seem to be more difficult than measuring the inclination.
Methods for measuring the azimuth relative to the geographical north utilize gyroscopic reference devices which have the disadvantage of being extremely sensitive to vibration such as occurs during drilling, as a result of which they must be installed after drilling is stopped and recovered before drilling is resumed.
It is for this reason that the present invention concerns a method of measuring the azimuth relative to the magnetic north, as known methods of this kind enable the measuring devices to be left in place while drilling proceeds. However, the magnetic sensors of these devices are responsive not only to the local terrestrial magnetic field, which they have to use as a reference, but also to stray fields such as those created by nearby ferromagnetic bodies. These devices must be accommodated in the lower part of the drill pipe string. The drill pipe string connects the drilling tool or motor to the surface and is formed by a series of tubes which are generally called "pipes". These pipes, the individual length of which is normally of the order of nine meters, are fabricated from a steel selected for its mechanical properties and price. This steel is ferromagnetic and creates stray magnetic fields.
To be more precise, failing any special precautions, the magnetic field in the vicinity of the lower end of the string of drilling pipes may be defined as the sum of three fields:
the local terrestrial magnetic field due to terrestrial magnetism and the influence of the terrain,
a stray magnetic field induced by the local terrestrial magnetic field in the drilling pipes, featuring a symmetry of revolution about the axis of these pipes, and
a possible further stray magnetic field associated with the pipes and possibly corresponding to a fixed magnetic assymetry of the drill pipe string or to the presence of a magnetized particle intended, for example, to define the angular position of the plane in which the deviation motor tends to curve the drill-hole. This angular position is known as the "tool face".
Known magnetic azimuth measuring methods eliminate the disturbing influence of such stray fields by replacing two or three of the drill collars (terminal drilling pipes) of the usual, that is ferromagnetic, type with special non-magnetic pipes (monel collars) in one of which the magnetic sensors are accommodated. The sensors are thus removed from sources of stray magnetic fields. However, given the mechanical forces that the drilling pipes have to transmit, the cost of these special pipes is high.
Also, certain known methods involve measuring three components of the magnetic field along three axes orthogonal to one another in pairs and entail temporary immobilization, that is to say temporary halting of rotation of the drill pipe string. As such rotation is generally essential to advancing the drilling, this entails a halt in such advance.
Another known method dispenses with a triple measurement along three fixed axes. It will now be described in outline by virtue of the fact that certain operations may be seen, generally speaking, as common to this known method and to the present invention. These common operations are:
producing a gravitational phase reference signal using a member responsive to gravity and a rotating system disposed in a tubular section of the drill pipe string constituting a measuring section and having an axis locally coincident with the drill-hole axis, this signal being synchronized to the angular position of said system relative to the vertical plane through said axis,
producing by means of a member fastened to said rotating system and responsive to the ambient magnetic field in said section a magnetometric signal synchronized to the angular position of said system relative to said magnetic field, said section consisting of a non-magnetic material so that said ambient magnetic field is not excessively different to the local terrestrial magnetic field,
measuring the phase of said magnetometric signal relative to said gravitational phase reference signal, and
using the result of this measurement to determine the azimuth of the measuring section axis relative to magnetic north.
This known method is described in French Pat. No. 2 068 829 (SAGEM).
To be more precise, this method entails stopping the rotation of the drill pipe string and the advance of drilling prior to the measurement operations. A complex rotating system is rotated about two axes by a motor. A pendulum device is provided with a weight which causes one of these rotation axes to assume a vertical position and which in this instance constitutes said member responsive to gravity. A member which forms part of the rotating system is responsive to the local magnetic field. It has a sensitivity axis and outputs an electrical pulse when the component of this magnetic field along this axis passes through a maximum value during rotation. This pulse constitutes a magnetometric signal as previously mentioned. A rotating member fastened to this member cooperates with a non-rotating member fastened to the weight in order to produce another electrical pulse which constitutes a gravitational phase reference signal as previously mentioned.
Apart from the fact of halting the progress of drilling, this known method has the disadvantages of employing a mechnically complex rotating system and necessitating the installation of non-magnetic drilling pipes.
An objective of the present invention is to enable the azimuth of a drill-hole inclined to the vertical to be measured relative to magnetic north accurately and continuously while drilling advances, without using costly non-magnetic drilling tubes or pipes or any complex special mechanical systems.
Another object of the present invention is to enable the position of a drilling tool to be known accurately, continuously and inexpensively during the progress of the drilling.